Technical Field
The present invention relates to technology for measuring distances using pulses of light.
Background Art
Techniques for measuring distances by using pulsed light are publicly known. One such technique is disclosed in Japanese Unexamined Patent Application Laid-Open No. 2013-011558. These techniques can be performed with higher distance measurement precision by using pulsed light with a short pulse width. In accordance with these techniques, a phase difference between distance measuring pulsed light that has been reflected from an object, of which distance is to be measured, and reference pulsed light that has propagated through an internal optical path of a device, is detected and is used for calculating the distance.
To process pulsed light that is detected, output of a light receiving element must be converted into a digital signal by an A/D converter. As the pulse width of the pulsed light is decreased, a sampling frequency of the A/D converter must be increased correspondingly. For example, to sample a waveform of pulsed light with a pulse width of 100 picoseconds, an A/D converter that can operate at a sampling frequency of several tens of GHz or higher and that can receive an analog signal with a bandwidth of at least 5 GHz or higher is required. Such an A/D converter is difficult to obtain, and even if such an A/D converter can be obtained, the A/D converter will be expensive and tends to consume large amounts of electrical power and generate large amounts of heat, thereby limiting usage conditions.
To cope with such problems, in accordance with one technique, a detection signal of pulsed light may be converted into a low frequency signal through a BPF (Band Pass Filter) and then may be converted into a digital signal by an A/D converter. In this technique, since a pulsed signal comes to have a damped oscillation waveform after passing through a BPF, a damped oscillation waveform of a distance measuring detection signal and a damped oscillation waveform of a reference detection signal can overlap on a time axis depending on a distance to be measured, and thus, a waveform may not be detected correctly.
To avoid such a problem, in accordance with one method, an external optical path for an optical path of measuring light and an internal optical path for an optical path of reference light may be made switchable, and distance measuring pulsed light that has propagated through the external optical path and reference pulsed light that has propagated through the internal optical path may be sampled at different timings. However, this method requires twice the time for measurement. Moreover, calculation errors can occur due to the time difference. The sampling precision of an A/D converter with a high sampling rate varies slightly at all times due to its heat generation and other effects. Thus, performing A/D conversions at different timings may cause very slight differences between digital data even though the digital data are obtained from the same data. In requiring a distance measurement precision of 2 to 3 mm or less, which is a standard of a typical surveying device, this very small difference may decrease precision. Accordingly, the method of processing the measuring light and the reference light at different timings is unsuitable from the viewpoint of the distance measurement precision.
To avoid such problems, the following method may be performed. In this method, two A/D converters are prepared, and a reference signal and a measuring signal are respectively made to pass through corresponding BPFs by using an RF switch. Then, the reference signal is sampled by one of the two A/D converters, and the measuring signal is sampled by the other A/D converter. However, as is known from the above description in which a very small time difference can cause variation in the calculation error, it is difficult to make characteristics of two A/D converters uniform. Thus, this method can cause decrease in the measurement precision as in the above method.